Organic electroluminescent displays, commonly called OLED devices for organic light-emitting diode devices, are typically coupled with active matrix (AM) circuitry in order to produce high performance displays. The basic OLED device has in common an anode, a cathode, and an organic electroluminescent medium sandwiched between the anode and the cathode. Such a display is disclosed in U.S. Pat. No. 5,550,066. However, in this bottom-emitting type of display where light is emitted downward through the substrate, the overall area that can emit light is limited by the presence on the substrate of Thin Film Transistors (TFT's) and other circuitry, which are opaque. Consequently the open area available for the light to emerge is reduced. The ratio of open area to that of the entire device area is called the Aperture Ratio (AR). Due to the reduction of the AR the display will run dim. To compensate for the reduced average brightness level, the drive current has to be increased subjecting the display to increased risk of operational degradation. This results in the lower AR devices having a shorter useable life than a device with a higher AR. In top-emitting device structures where the light is made to emerge through top surface away from the substrate and TFT circuitry, the AR is significantly higher than the conventional bottom emitting devices.
Therefore, much work has been done to produce AM devices, which are top- or surface-emitting. This configuration has potential to improve display performance compared with bottom-emitting OLEDs by: 1) increasing the aperture ratio, therefore allowing the pixel to operate at a lower current density with improved stability; 2) allowing more complex drive circuitry to enable better control of pixel current, leading to enhanced display performance (uniformity, stability); 3) enabling lower mobility materials, i.e., amorphous silicon, to be considered for TFT fabrication; and 4) allowing schemes for increasing the emission out coupling (increased efficiency) that are not available for the bottom-emitting format. A design for top-emitting OLEDs utilizes a reflective metallic anode and a semi-reflective metallic cathode as the top electrode. These metallic materials contribute to a microcavity effect within the devices that limits the optical qualities. Highly transparent materials (e.g., indium-tin-oxide (ITO)) are being considered to replace the semi-reflective cathodes; however, known deposition methods for these materials are not compatible with the said structure designs, and these highly transparent materials are less electrically conductive than the semi-reflective materials. The fact that these materials are less electrically conductive makes implementing a highly transparent cathode in a large display difficult because of the greater distance that the current must travel.
In U.S. Pat. No. 6,420,031, assigned to The Trustees of Princeton University, a class of low reflectivity, high transparency, non-metallic cathodes useful for a wide range of electrically active, transparent organic devices is disclosed. The representative embodiment of this invention employs ITO as the electrically conductive non-metallic layer and a phthalocyanine compound such as ZnPc or CuPc as the electron-injecting interface layer. The low-resistance electrical contact is formed only when the ITO is deposited onto the organic layer and not when the organic layer is deposited onto the ITO layer. The CuPc layer functions as: 1) a protection layer, preventing damage to the underlying organic layers during the ITO sputtering process; and 2) an electron-injecting region, functioning in combination with the ITO layer to deliver electrons to the adjacent electron transporting layer. This solution for delivering a highly transparent cathode for use in an OLED is insufficient because it: 1) does not optimize the electron injection into the electron transport layer; and 2) uses materials unsuitable for full color devices.
Destructive light interference can result from micro-cavity effects within a top-emitting OLED with a reflective cathode and can cause color distortion when the top-emitting OLED is viewed from oblique angles. The emission from microcavity devices is characteristically directional. The emission is shifted to shorter wavelength and the intensity falls rapidly with viewing angle. (See, for example, N. Takada, T. Tsutsui, and S. Saito Appl. Phys. Lett. 63 (15) 2032 (1993) “Control of emission characteristics in organic thin film electroluminescent diodes using an optical microcavity structure”.) Additionally, much less light is absorbed in the transparent ITO cathode compared to the semitransparent cathode of a top-emitting microcavity OLED. This means that less power can be used to emit the same level of luminance, or the same power can be used to emit a higher level of luminance. Thus, there exists a need for a top-emitting OLED with a highly transparent cathode.
Deposition methods for highly transparent cathodes (like those comprising indium tin oxide) in top-emitting OLEDs generally involve sputter deposition. Sputter deposition is a preferred method of depositing these cathodes because 1) the method allows optimization of the composition during film deposition for maximization of transparency and conductivity, and 2) the deposition method is compatible with mass manufacturing. However, sputtering can be damaging to the electron transport layer (ETL) within OLED devices. (See L. S. Liao, L. S. Hung, W. C. Chan, X. M. Ding, T. K. Sham, I. Bello, C. S. Lee, and S. T. Lee, “Ion-beam induced surface damages on tris-(8-hydroxyquinoline)aluminum”, Appl. Phys. Lett. 75, 1619 (1999).) This damage reduces the intensity of the emission and may additionally render the pixels permanently inoperable. Thus, there exists a need to protect the ETL during manufacturing of an OLED.